In the fields of construction machines/vehicles, other types of machines/vehicles, etc., fluid pressure devices, such as fluid pressure cylinders, are used as driving actuators for working machines. In driving such a fluid pressure device, pressurized fluid having a controlled direction and flow rate is supplied to a given fluid pressure port via a flow rate control valve. Examples of the fluid pressure devices of the aforementioned type include the ones disclosed in Japanese Patent Application Laid-open Publication No. HEI-9-177137 (hereinafter referred to as “patent literature 1”) and in Japanese Patent Application Laid-open Publication No. 2000-213506 (hereinafter referred to as “patent literature 2”), and various types of pilot valves are used as the flow rate control valve. The flow rate control valve is constructed as a servo valve, and a position of a spool of the flow rate control valve is detected by a position detector so that spool position detection data is fed back to an actuator that drives the spool. One example of a spool-type flow rate control valve having such a position detection function is disclosed in Japanese Patent Application Laid-open Publication No. 2002-297243 (hereinafter referred to as “patent literature 3”).
Today, various types of position detectors are known, such as an electric resistance type, electromagnetic type, optical type, etc. The position detector of the electric resistance type using a potentiometer is not satisfactory in terms of durability because it has mechanical contacts. The position detector of the optical type has a drawback of requiring cumbersome maintenance if it is used in an ambient environment that tends to be easily contaminated, although it can detect a position in a contactless manner. Further, the position detector of the electromagnetic type using an AC-energized coil has advantages that it can detect a position in a contactless manner and is durable even in an easily-contaminated environment. However, with the position detector of the electromagnetic type, how to compensate for coil impedance variation, which occurs due to a temperature characteristic, so as to achieve position detection with high accuracy becomes an important issue. Further, no matter of what type the position detector may be, it is desirable that the position detector be of simple construction and require only low manufacturing cost.
Further, flow rate control valves of various sizes are available depending on different purposes of use. If a maximum variation amount of a spool, which is an object of detection, differs, the conventionally-known spool position detection device must use a position sensor having a detectable range corresponding to the different maximum variation amount. Thus, in the past, it was necessary to prepare different position detection devices for various models of flow rate control valves of various sizes.